A compact, on-chip optical modulator is a key component for enabling optical interconnect systems on a microelectronic chip for various applications, such as optical communications, radio-frequency (RF) waveform generation, and optical signal processing. An on-chip optical modulator made of silicon is referred to as a silicon optical modulator. A silicon optical modulator is also a key component of a silicon optical transceiver.
There are at least four parameters to characterize a silicon optical modulator: Vπ, the voltage swing required to achieve a π phase shift; insertion loss; modulation speed; and modulation efficiency. A small Vπ indicates that a small voltage induces a phase shift, so a silicon optical modulator with a small Vπ consumes relatively less power. Insertion loss is defined as the power loss due to the insertion of the silicon optical modulator into a system and is related to the length of the silicon optical modulator. For example, a longer silicon optical modulator has more insertion loss than a shorter silicon optical modulator. Modulation speed corresponds to the maximum data rate of the RF signals that the silicon optical modulator can modulate with. Normally, Vπ is a trade-off with insertion loss and modulation speed because a longer silicon optical modulator allows for a smaller Vπ, but results in a slower modulation speed and higher insertion loss. Modulation efficiency is related to the product of Vπ and L, where L is the length required to achieve a π phase shift. A high modulation efficiency corresponds to a small product of Vπ and L.
A silicon optical modulator may allow for integration into existing conventional complementary metal-oxide-semiconductor (CMOS) processing platforms. However, early silicon optical modulators, which employed a lateral p-n junction and a traveling wave modulator architecture following the design of traditional modulators fabricated from group III-V compounds such as indium phosphide (InP), gallium arsenide (GaAs), and lithium niobate (LiNbO3), suffered from high optical insertion loss, high power consumption, and a limited optical extinction ratio. In addition, silicon has a reasonable free-carrier plasma dispersion effect, but has less electro-optical modulation efficiency than group III-V and II-III materials.